(a) Field of the Invention
This invention relates to a process for producing a light-transmitting element composed of a synthetic resin. More specifically, this invention relates to a process for producing a light-transmitting element of a synthetic resin, of which refractive index gradually decreases from its central portion toward its marginal portion.
(b) Description of the Prior Art
Processes for the production of light-transmitting elements of synthetic resins are disclosed in U.S. Pat. No. 3,955,015 Japanese Patent Publication No. 30,301/1979 and Japanese Laid-Open Patent Publication No. 149,004/1981.
U.S. Pat. No. 3,955,015 discloses a method of manufacturing a transparent light-transmitting element of synthetic resin, which comprises producing a prepolymer from a monomer A, imparting a desired form to the prepolymer, contacting the prepolymer article with a monomer B to diffuse the monomer B in the prepolymer article to form a concentration gradient of the monomer B in the article decreasing from the surface to the interior of the article, and then subjecting the prepolymer and the monomer B to post-polymerization to complete the polymerization.
Japanese Patent Publication No. 30,301/1979 and Japanese Laid-Open Patent Publication No. 149,004/1981 disclose methods of producing light-transmitting elements of synthetic resin which have a refractive index distribution represented by the following equation (1) EQU N=N.sub.o (1-1/2A r.sup.2) (1)
wherein N.sub.o is the refractive index of the central axis of the light-transmitting element, N is the refractive index of a point away from the central axis of the light-transmitting element by a distance r, and A is its refractive index distribution constant.
However, polymeric articles obtained by the methods disclosed in these Japanese patent documents have the refractive index distribution of equation (1) only at their central portions, and to use them as light-transmitting elements, their marginal portions must be removed. The reasons are as follows:
These methods are based on the utilization of the fact that the composition of a copolymer varies with the progress of polymerization. Thus, according to these methods, monomers M.sub.1 and M.sub.2 are selected which when converted into polymers, have different refractive indices from each other and form a transparent copolymer; and then copolymerization conditions are selected such that copolymerization is started at a predetermined part of a monomeric mixture maintained in a predetermined form and then the resulting copolymer is continuously precipitated in the reaction system, thereby producing a light-transmitting element having a refractive index gradient.
The Japanese patent documents describe a method for producing a light-transmitting element of synthetic resin having a refractive index gradient, which comprises maintaining a mixture of monomers M.sub.1 and M.sub.2 in a predetermined form such as a cylindrical form, the monomers M.sub.1 and M.sub.2 having refractive indices differing from each other by 0.005 or larger and also having a Q value, defined by the following equation, of at least 1.1 or not more than 1/1.1, EQU Q={r.sub.1 (M.sub.1 /M.sub.2).sub.m +1}/{(M.sub.1 /M.sub.2).sub.m +r.sub.2 }(2)
wherein r.sub.1 and r.sub.2 are the monomer reactivity ratios of the monomers M.sub.1 and M.sub.2 in copolymerization reaction and (M.sub.1 /M.sub.2).sub.m is the mixing ratio in moles of M.sub.1 /M.sub.2 ;
and thereafter subjecting the monomeric mixture in the predetermined form to copolymerization conditions which are non-uniform in regard to place, whereby a copolymer having an M.sub.1 /M.sub.2 ratio different from the aforesaid mixing ratio is locally formed only in a predetermined part of the mixture, for example only a peripheral portion of the mixture in cylindrical form, and thereafter from that part toward another part such as a central part, the copolymerization proceeds gradually, thereby to provide within the interior of the copolymer such a concentration gradient that the ratio of the M.sub.1 content to the M.sub.2 content gradually changes from said predetermined part toward said other part.
As a combination of monomers M.sub.1 and M.sub.2, the patent documents cite a combination of methyl methacrylate and vinyl benzoate, and also combinations of alkyl methacrylates and vinyl phenylacetates. Light-transmitting elements obtained from such combinations of monomers have refractive index distributions of equation (1) only near their central axis, and at their peripheral portions, the gradient of refractive index is very gentle. This is because the refractive index of the resulting copolymer gradually increases with the progress of polymerization, and the increase is gentle in the initial stage, but in the latter half of the polymerization process, the increase is abrupt. In order, therefore, to obtain rod-like convex lenses from light-transmitting elements obtained by these methods, it is necessary to use only those portions which are near their center by cutting off their peripheral portions.